ECG Blog #457 — Is Anything Conducting?

I was sent the ECG shown in Figure-1 — told only that that it was from an acutely ill patient on a ventilator, who was being evaluated for bradycardia. His providers thought this rhythm was complete AV block.

QUESTIONS: 

• How would you interpret the ECG in Figure-1?
• Is the rhythm complete AV block?

Figure-1: The initial ECG in today's case. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Thoughts on Figure-1: 

The "Short" Answer to the question of whether today's rhythm represents complete AV block is no! The "good news" — is that I knew within seconds that today's rhythm was unlikely to represent complete AV block — because there is group beating!

• Most of the time when there is complete AV block — the ventricular response will be regular (or at least almost regular). This is because most escape rhythms (be they from the AV node; the His; the ventricles) — tend to be surprisingly regular — unless altered by hyperkalemia, acidosis or other toxicity.
• So, while exceptions are always possible — the presence of the regular irregularity (ie, group beating, in the form of alternating short-long intervals of comparable duration) — makes it highly unlikely that there is no conduction, as would be expected if 3rd-degree AV block was present.
• NOTE: If there is any doubt on visual inspection about the regular irregularity in Figure-1 — using calipers allows you within seconds to verify comparable duration for the RED arrow R-R intervals — and comparable duration for the slightly longer BLUE arrow R-R intervals (shown in Figure-2).
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• PEARL #1: The presence of group beating — should always suggest the possibility of Wenckebach conduction. While there are other potential causes of group beating (ie, atrial bigeminy; blocked PACs, etc.) — recognizing this finding (as we see in Figure-2) is a "tip-off" to be on the alert for possible 2nd-degree AV block, Mobitz Type I. And, as soon as we recognize group beating — We know that the rhythm is probably not complete AV block!

Figure-2: Alternating short-long R-R intervals (highlighted by RED and BLUE markers of comparable duration) — indicate group beating in today's rhythm.

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The "Longer" Answer: What is the Rhythm?

As always — I favor assessment of the cardiac rhythm by the Ps, Qs, 3R Approach as a time-efficient, user-friendly systematic way to recall the 5 KEY Parameters (See ECG Blog #185 for review of this concept).

Returning to Figure-1 — I look for the Ps, Qs and 3Rs:

• P waves are clearly present.
• The QRS complex is clearly narrow in all 12 leads.
• The rhythm is not Regular — because as we have already determined there is group beating.
• As a result of this group beating — the Rate is not constant, but since R-R intervals are between 4-to-6 large boxes in duration — the overall heart rate is reasonable (between 55-70/minute).
• Which leaves us with having to assess the 5th Parameter — which is "Related" (ie, Whether any of the P waves that are present are "related" and therefore conducting any of the neighboring QRS complexes?).

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In Figure-3 — We look closer at the Ps, Qs and 3Rs:

• RED arrows in Figure-3 show that there is an underlying regular sinus rhythm (upright P waves with minimal variation in the P-P interval thoughout the long lead II rhythm strip).
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• PEARL #2: Note that there are more P waves than QRS complexes in Figure-3 (ie, a total of 15 RED arrow P waves — but only 10 QRS complexes). This means that at least some of these P waves are not conducting — and, since P waves are regular (ie, All P waves are "on-time" — which means there are no blocked PACs) — this tells us that the reason some of these "on-time" P waves are not conducting, is that there is some form of 2nd-degree AV block!

PEARL #3: Now step back for a moment — and take another LOOK at the long lead rhythm strip in Figure-3. Doesn't the simple act of labeling all P waves in today's rhythm facilitate assessment of the 5th Parameter? Assessing this 5th Parameter is KEY to solving today's rhythm = Are any of the P waves in Figure-3 conducting?

• HINT: To answer this question — LOOK in Figure-3 at the PR intervals in front of each beat in the long lead II rhyhm strip. Are any of these PR intervals the same?

Figure-3: I've labeled all P waves in today's rhythm with RED arrows.

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ANSWER: Are any of the P waves in Figure-3 conducting?

In Figure-2 — We saw that there is group beating with alternating shorter and slightly longer R-R intervals.

• Figure-4 highlights that each of the PR intervals that end the slightly longer R-R intervals in today's rhythm are the same! (ie, BLUE arrows in front of beats #1,3,5,7 and #9 in Figure-4 show that these PR intervals are all identical = 0.40 second). This proves that each of these P waves in front of beats #1,3,5,7,9 is conducting!
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• PEARL #4: The findings of group beating + a regular underlying sinus rhythm in which there are more P waves than QRS complexes — but in which each short "pause" in the rhythm (ie, each of the slightly longer R-R intervals in Figure-4) ends with a sinus P wave that conducts with the same PR interval — all but confirms that today's rhythm is some form of 2nd-degree AV Block, Mobitz Type I (ie, AV Wenckebach).

The findings noted in PEARL #4 constitute several of the "Footprints" of Wenckebach (discussed more in ECG Blog #251).

• While at this point in my assessment — I was all-but-certain that today's rhythm represented some for of Mobitz I (which really is all that we need to know for optimal clinical management) — I had not yet demonstrated cycles with progressive increase in the PR interval until an on-time sinus P wave is dropped (as should be seen with typical AV Wenckebach).
• To prove my theory — I needed to construct a laddergram (See below).

Figure-4: BLUE arrows highlight that the PR intervals in front of beats #1,3,5,7,9 are identical — therefore confirming that each of the P waves in front of these beats is conducting!

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What about the 12-Lead ECG?

At this point (before looking closer at today's rhythm with use of a laddergram) — We should consider the 12-lead ECG (shown above the long lead II rhythm strip in Figure-3 that I show again below).

• There is low voltage in the limb leads (QRS amplitude ≤5 mm in all limb leads — with potential causes of low voltage discussed in more detail in ECG Blog #272).
• As already noted — the QRS is narrow in all 12 leads.
• Considering the slow rate — the QTc does not look to be prolonged.
• The frontal plane axis is normal (about +60 degrees).
• There is no chamber enlargement.
• There are QS waves in leads V1-thru-V4. This suggests there has been anterior infarction at some point in time. That said — there is non-specific ST-T wave flattening in many leads that does not appear to be acute.

Another LOOK at Figure-3: What about the 12-lead ECG?

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The LADDERGRAM: 

Returning to a more in-depth look at today's rhythm — the BEST way to prove that this rhythm is 2nd-degree AV block of the Mobitz I (AV Wenckebach) Type — is to construct a laddergram. I illustrate this in Figures-5 thru -10:

Figure-5: Laddergram STEP-1. It is usually easiest to begin a laddergram by filling in the Atrial Tier. BLUE arrows show the onset of P waves as my reference point for drawing in atrial activity. Because conduction through the atria is generally rapid — I drew in near-vertical lines in the Atrial Tier.

Figure-6: Laddergram STEP-2. I next fill in the Ventricular Tier.
 BLUE arrows show the onset of each QRS complex as my reference point for each of the QRS complexes in this tracing. 

Note that I drew in near-vertical lines for each of the 10 QRS beats in the Ventricular Tier. (If the 12-lead ECG were to show bundle branch block with a wide QRS complex — I would then draw more angled lines in the ventricular tier to reflect slower conduction as impulses passed through the ventricles).

KEY Point: The "EASY part" for constructing most laddergrams consists of these first 2 STEPS (as shown in Figures-5 and -6). Now the challenge begins — which is trying to "solve" the laddergram by figuring out which of the P waves in the Atrial Tier are being conducted to the ventricles.

Figure-7: We are now ready to begin solving the laddergram. I do this by connecting those P waves from the Atrial Tier — to those QRS complexes that I feel it most logical for these P waves to conduct to (BLUE lines that I've drawn within the AV Nodal Tier).

Figure-8: It seems logical that the next P waves to conduct to the ventricles are those that are highlighted by BLUE arrows in Figure-8. It should now be easy to see that conduction of this 2nd P wave in each group takes a bit longer to be conducted, which is the principal characteristic of Wenckebach conduction (BLUE lines that I've drawn within the AV Nodal Tier).

Figure-9: By the process of elimination — this means that the remaining YELLOW arrow P wave in each group is not conducted (ie, there are no unconnected QRS complexes left). This therefore "completes" the laddergram — by the "butt end" that I've added to the remaining YELLOW arrow P waves.

Figure-10: Final laddergram — with P waves color-coded.
Within each group, the 1st P wave (light BLUE arrows) conducts with a prolonged PR interval = 0.40 second. The 2nd P wave (dark BLUE arrows) conducts with an even longer PR interval — and then the 3rd P wave in each group (YELLOW arrows) is non-conducted — after which, the cycle begins again with the next light BLUE arrow. Thus, the rhythm = 2nd-degree AV Block, Mobitz Type I (AV Wenckebach) with 3:2 AV conduction (because there are 3 P waves for every 2 P waves that are conducted).

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Acknowledgment: My appreciation to Jean Max Figueiredo (from Iguaçu, Brazil) for the case and this tracing.

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Related ECG Blog Posts to Today’s Case:

• ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
• ECG Blog #185 — Reviews my System for Rhythm Interpretation, using the Ps, Qs, 3R Approach.
• ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 100 laddergram cases — many with step-by-step sequential illustration).
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• ECG Blog #192 — The 3 Causes of AV Dissociation.

• ECG Blog #191 — Reviews the difference between AV Dissociation vs Complete AV Block.
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• ECG Blog #389 — ECG Blog #373 — and ECG Blog #344 — for review of some cases that illustrate "AV block problem-solving".
• ECG Blog #251 — Reviews the concepts of Wenckebach periodicity and the "Footprints" of Wenckebach.
• ECG Blog #164 — Reviews a case of typical Mobitz I 2nd-Degree AV Block (with detailed discussion of the "Footprints" of Wenckebach). 
• ECG Blog #236 — for an ECG Video Pearl on the 3 Types of 2nd-degree AV block.
• ECG Blog #344 — thoroughly reviews the Types of 2nd-degree AV block (Mobitz I vs Mobitz II vs 2:1 AV Block).
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• ECG Blog #63 — Mobitz I, 2nd-degree AV block with junctional escape.
• ECG Blog #195 — reviews Isorhythmic AV Dissociation.
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• ECG Blog #267 — Reviews with step-by-step laddergrams, the derivation of a case of Mobitz I with more than a single possible explanation.
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• ECG Blog #405 — ECG Video presentation that reviews the distinction between AV Dissociation vs Complete (3rd-degree) AV Block (For a LINKED Contents to this ECG Video — Click on MORE in the Description under the video on YouTube).